1. Field of the Invention
The invention relates to a method of manufacturing a colour display tube in which magnetic poles are provided in or around the neck of the envelope and around the paths of the electron beam extending substantially parallel to the axis of the tube, said magnetic poles generating a permanent multipole magnetic field to correct the occurring convergence, colour purity and frame defects of the colour display tube, said poles being formed by magnetizing a configuration of a magnetizable material which is provided around the paths of the electron beams, said configuration being magnetized by energizing a multipole coil unit by means of a combination of currents with which a static multipole magnetic field is generated and the magnetization is produced by means of a decaying alternating magnetic field which initially drives the magnetizable material on both sides of the hysteresis curve into saturation.
The invention also relates to a device for carrying out said method.
2. Description of the Related Art
Such a method and device are disclosed in U.S. Pat. No. 4,220,897.
In a colour display tube of the "in-line" type three electron guns are placed in the neck of the tube so that the axes of the three guns are situated substantially in one plane, the axis of the central electron gun coinciding substantially with the axis of the display tube. The two outermost electron guns are situated symmetrically with respect to the central gun. In a colour display tube of the "delta" type, three electron guns are provided in a triangular arrangement in the neck of the tube. The points of intersection of the gun axes with a plane perpendicular to the tube axis constitute the corners of an equilateral triangle. As long as the electron beams generated by the electron guns are not deflected, the three electron beams both in tubes of the "in-line" type and of the "delta" type, must coincide in the centre of the display screen (static convergence). However, because during the manufacture of the display tube deviations from the frame shape, the colour purity and the static convergence occur, for example, the electron guns are not sealed quite symmetrically with respect to the tube axis, it must be possible to correct said deviations. Such a colour display tube of the "in-line" type in which this is the case is disclosed in U.S. Pat. No. 4,211,960 which may be considered to be incorporated herein. Said patent discloses a colour display tube in which the said deviations are corrected by magnetizing a ring of a magnetizable material as a result of which a static magnetic multipole is formed around the paths of the electron beams. Said ring is provided in or around the neck of the tube. In the method described in said U.S. Pat. No. 4,211,960 the colour display tube is actuated after which data regarding the value and the direction of the convergence errors of the electron beams are established with reference to which the polarity and strength of the magnetic multipole are determined which are necessary for the correction of the frame, colour purity and convergence errors. The magnetization of the configuration, which may consist of a ring, a band or a number of rods or blocks arranged around the electron paths, is carried out in the manner described in the opening paragraph in which a multipole is obtained by one overall magnetization. The magnetization achieved by this single step often is not good and one or more magnetization steps must still be carried out. This is the result of the spreading in the magnetic hardness of the material of the configuration to be magnetized, the spreading in the coupling between the configuration to be magnetized and the magnetizing coils, the spreading in the form of the the configuration to be magnetized and the location of other metal components in the proximity of said configuration. Each magnetization step requires an amount of time for controlling the desired correction. During the magnetization process the voltages at the electrodes of the electron gun and the currents through the deflection coils must also be switched off.